Well I just went onto Finnair "Manage my bookings" link and updated my FF# on my AB booking. It accepted it. A few minutes later I went onto AB's website and checked my booking and it now shows my TopBonus number with indication that I get a free checked item and managed to select a free seat as well.
I find their website a bit weird though, I selected my free seat and still have to go through entering payment details before I can confirm the free seat....